Propulsion systems for work machines typically comprise one or more power units, such as gas turbines and engines. Such work machines include vehicles, such as marine vessels, aircraft or land vehicles. The power output from the power units may, for example, provide the required loads to house loads, operator quarters or work tools. Exemplary work tools include cranes, drills, bow or stern thrusters, grapples, buckets and backhoes. If the work machine is a vehicle, the power may provide a thrust to move the vehicle.
The range of potential power output from the power unit may be selected in order to meet the expected loads required in the operating profiles of the work machine. There are also environmental and cost reduction benefits in operating such power units efficiently. In particular, the power unit may be selected to operate at maximum efficiency at the vehicle cruising speed, which is the speed at which the vehicle moves for the majority of the time it is operational.
By way of example, marine tankers have an operating profile in which the majority of time is spent at cruising speed. Such tankers may not include a transmission, or may have a relatively simple transmission, provided by a shaft transferring power between the power unit and propeller. It may therefore be fairly simple to select an appropriately sized power unit to operate efficiently at cruising speed. However, it may not be possible to operate the power unit at maximum efficiency if the vessel's operating profile is relatively varied.
As another example, the operating profile of a tugboat includes substantial time spent maneuvering, which requires low power; substantial time spent travelling at medium speed, which requires medium power; and substantial time spent pushing or towing other ships, which requires high power. It has been found that tugboats can spend the majority of their operating time at below 20% of the rated load of the power unit providing thrust.
Marine vessels typically comprise either a mechanical or an electrical propulsion system. An electrical propulsion system may comprise a one or more power unit(s) driving an electric generator, which feeds power to at least one electric motor to drive one or more propeller(s). FIG. 1 illustrates a prior art ship 10, in this case an anchor handling tug supply vessel, comprising a mechanical propulsion system 11. The mechanical propulsion system typically comprises a power unit 12, such as a diesel engine, rotating a drive shaft 13. The drive shaft 13 supplies power to a gearbox 14, the output of which is connected to a propeller shaft 15. A propeller 16, which in this example is a fixed propeller, is connected to the propeller shaft 15 to provide a thrust to the ship 10. The power unit 12 may also supply power to an electric generator, which may be used to provide power to the operator quarters 17, bow thrusters 18, stern thrusters 19 and crane 20.
As illustrated in FIG. 2, the gearbox 14 is typically relatively simple and may comprise an input gear 21 attached to the drive shaft 13 and an output gear 22 attached to the propeller shaft 15. The input and output gears 21, 22 may interlock in a countershaft arrangement. The output gear diameter 23 may depend upon the gear ratio required of the gearbox 14. Therefore, if a large reduction in rotational speed between the drive shaft 13 and propeller shaft 15 is required, the output gear diameter 23 may be relatively large. As the output gear 22 is typically located in a keel 24 of the ship 10, the keel 24 may have to be relatively wide to accommodate the output gear 22. A relatively wide keel 24, however, may have a negative impact upon the operational efficiency of the ship 10 due to, for example, increased drag. If the gearbox 14 has a high reduction gear ratio, the width of the keel 24 will depend upon the largest output gear diameter 23.
Marine vessels may also comprise variable pitch propellers, rather than fixed pitch propellers, to improve operational efficiency. The power unit may be run at a more efficient power output whilst the propeller pitch is varied in order to change the ship speed. However, such variable pitch propellers may have drawbacks, for example high cost and complexity, in comparison with fixed pitch propellers. Furthermore, when a hydraulic actuation mechanism is utilised to vary the propeller pitch, oil from the actuation mechanism may cause water pollution. For example, the variable pitch propeller may require a larger hub to house the actuation mechanism which results in an increase in drag in comparison to fixed pitch propellers. In addition, the efficiency of the variable pitch propeller may only be at an optimum at a certain pitch angle and may reduce significantly as the pitch angle is varied. Also, it is not possible to effectively fit a shroud or duct around a variable pitch propeller due to the variation in propeller diameter as the pitch is varied. Shrouded or ducted fixed pitch propellers may have significant efficiency advantages in some applications. Shrouds may be used to improve propeller output efficiency by reducing radial losses and improving axial thrust. However, it may be difficult to provide the same flexibility as provided by a variable pitch propeller, whilst including a shroud.
A hybrid propulsion system may be utilised to improve flexibility, redundancy and efficiency. US-A-2010/0144219 discloses a marine vessel hybrid propulsion system comprising multiple internal combustion engines, multiple electrical generators, multiple electrical motors and multiple propellers. The engines may provide power to the generators and/or to the propellers. The generators may provide energy to be stored in a battery or directed to the motors. The motors may provide energy to the propellers.
However, US-A-2010/0144219 does not disclose how the outputs of the motors and engines can be combined to provide output to a single propeller. Furthermore it does not disclose how the speed of the propeller is controlled. This prior art system also does not provide means for the engines to supply power to the generators whilst both the engines and motors supply power to the propellers. Therefore, the battery may run out of energy to drive the motors. The system also does not provide flexibility in the propeller speed when the engine is run at full operational speed. Furthermore, no means are provided for increasing the propeller speed beyond the speed at which the motor and engines are providing maximum power to the propellers.